The present invention generally relates to amplification of optical signals in optical fibers and other waveguide devices, and, more specifically, to amplification of light signals at a wavelength of 1.31 .mu.m.
Currently, optical fibers are being installed throughout the world for communication purposes. Where long distances must be traversed, as with trans-oceanic cables, it is a common problem to minimize the number of active amplifiers for the line. It is for this reason that most long distance telecommunication lines are operated at a wavelength of 1.55 .mu.m. At this wavelength, attenuation in signals in silica optical fiber is reduced, allowing for the use of fewer amplifiers per unit of distance. Erbium-doped optical fiber amplifiers which recently have been developed are sufficiently efficient and small enough for telecommunication applications. However, 1.55 .mu.m wavelength systems do suffer from relatively high signal dispersion, which results in temporal broadening of the light pulses.
In contrast, signal dispersion at 1.31 .mu.m wavelength in silica optical fibers is much improved, so that signals can be transmitted at the greatest temporal rate, making it also attractive for local area computer networks, subscriber cable television networks, and high-speed computing networks. Unfortunately, there has been little success in the development of practical 1.31 .mu.m wavelength optical amplifiers.
Because of the benefits of the 1.31 .mu.m wavelength, systems utilizing that wavelength have been installed by most telecommunication industries in the United States and in the United Kingdom. For amplification, these systems use a hybrid opto/electronic regenerator which decodes the optical information with sensors and uses electronics to command LEDs or lasers to retransmit the information down the next section of optical fiber. These regeneration systems are complex, bulky and costly, and have a much lower level of efficiency, reliability and capacity than is desired for modern communication systems.
Work on more practical optical amplifiers for the 1.31 .mu.m wavelength has been proceeding for some time without a great deal of success. Amplifiers for this wavelength have been demonstrated in the laboratory, but have not proven to be downscable to the size necessary for telecommunication applications. In doing this work, however, it has been recognized that proper doping of the optical fibers can tailor fibers to amplify at a desired length. For example, Praseodymium ions (Pr.sup.+3) have been used with limited success to dope standard ZBLAN.RTM. (fluoro-zirconate) fiber glass. The major problem with this fiber is that it must be pumped at a wavelength of 1.02 .mu.m, a wavelength for which no economically feasible high-power diode laser currently exists.
Other work has shown that Ytterbium ions (Yb.sup.3+) can be added as a codopant with Pr.sup.3+ to provide energy transfer with Pr.sup.3+. This system can be pumped at 890 nm, a reasonable diode laser wavelength, and energy is transferred to the Pr.sup.3+ ions to provide gain at 1.31 .mu.m. Regrettably, this system has not proven to be as successful as hoped because of inefficiency of the energy transfer between the dopants, and because of problems with the energy absorption of the Yb.sup.3+ ions at this wavelength.
It is therefore an object of the present invention to provide practical apparatus for the amplification of signals at the 1.31 .mu.m wavelength.
It is another object of the present invention to provide a 1.31 .mu.m amplification system which can be pumped by readily available lasers.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages o the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.